The bug seems to be triggered by the use of a symbolic function (the one
created by function('chi')).
It is now tracked at
https://trac.sagemath.org/ticket/27492
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Thanks for the report. I confirm the bug in Sage 8.7.beta7.
What is strange is that ddChi seems a perfectly valid symbolic expression:
sage: ddChi = ddChi.expand(); ddChi
2*diff(chi(r, theta, phi), r)/r + cos(theta)*diff(chi(r, theta, phi), theta
)/(r^2*sin(theta)) + diff(chi(r, theta, phi), theta
